How we lost karma?

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You can track the evolution of your karma in your profile. Put your mouse on a question title and you will get some information on your loss of karma.

More information is available in the ROS Answers FAQ:

    How does karma system work?

    When a question or answer is upvoted, the user who posted them will gain some points, which are called "karma points". These points serve as a rough measure of the community trust to him/her. Various moderation tasks are gradually assigned to the users based on those points.

    For example, if you ask an interesting question or give a helpful answer, your input will be upvoted. On the other hand if the answer is misleading - it will be downvoted. Each vote in favor will generate 10 points, each vote against will subtract 10 points. There is a limit of 200 points that can be accumulated for a question or answer per day. The table below explains reputation point requirements for each type of moderation task.

    1    upvote
    3    post a link
    1    add comments
    50   downvote
    25   accept own answer to own questions
    25   open and close own questions
    100  retag other's questions
    75   edit community wiki questions
    1000 edit any answer
    200  delete any comment

If your questions/answers are not relevant enough, some people will probably downvote. Try to read ROS tutorials and other ROS questions before asking one, so that people do not end up feeling that you did not search anywhere before asking your question. Same goes for answers.